1. Field of the Invention
The present invention relates to brake-friction materials used in cars and the like, and especially relates to brake pads.
2. Prior Art
Heretofore, two kinds of brake-friction materials, asbestos brake-friction material and non-asbestos brake-friction material made of steel fiber, ceramic fiber or the like, are known as the base materials in cars and the like.
One example of the manufacture of non-asbestos brake-friction materials is the hot press process. First, steel fiber base material is mixed with graphite as a lubricant, barium sulfate and the like as a filler, and phenolic resin as a bonding material. The mixture is stirred to provide uniform mixing. Subsequently, the uniform mixture is heated and formed to the desired shape by compression molding. Hereafter, it is called a "pad" when the brake friction material formed by the above described process is arranged in the braking system.
The temperature of the brake-friction material described above reaches several hundred degrees centigrade when the braking system is successively applied on a downward slope. In order to protect the brake-friction material from damage at the high temperature, the resin bonding material must be heat resistant.
For example, poor bonding strength between the resin bonding material and graphite lubricant is one of the reasons of wear of the brake-friction material. We, the present applicants, have investigated the various conditions under which the brake-friction material functions in order to determine the characteristics mentioned above.
One of the ways to prevent the brake-friction materials from suffering the above mentioned damage and to improve abrasion resistance of the pad is to increase the amount of graphite included in the brake-friction material. The wear of the brake-friction material will decrease if the amount of graphite included in the brake-friction material is increased. However, as the abrasion resistance of the graphite-rich brake-friction material is improved, its shear strength becomes lower, because the bonding strength between graphite and phenolic resin is weak.
When the amount of graphite included in the brake-friction material is excessive, the heat conductivity of the pad increases and causes vapor lock phenomenon of hydraulic system.
In order to adjust the amount of graphite included in the brake-friction material, antimony(III) sulfide having superior heat resistance to graphite is added. When phenolic resin attains a high temperature and gas is generated for example methane and the like by heat decomposition, the gas enters between the pad and the disk. As a result, fade phenomenon (braking power is decreased) is likely to occur. If the porosity of the pad is made large, the pores will catch the gas generated from phenolic resin.